Mrs May said while they were referred to in a report as “foreign fighters” she viewed them as “potential terrorists, some of whom will be of a Jihadi mind-set”.

The Home Secretary also said the Government has given the police greater powers to deal with the kind of firearms attacks seen in the Westgate shopping mall in Kenya.

She said the Government had tightened security to address the threat from “potential firearm attacks” in the wake of the attacks on a luxury hotel in Mumbai in 2008.

She said: “We have increased the ability of police to deal with those sort of attacks, the number of armed response vehicles, the number of specially-trained firearm officers, but of course we continue to look at these issues to continue to ensure we are doing all we can to keep people safe.

“One element of our counter-terrorism strategy is the protect element, and that is about working with the private sector to ensure suitable protection are in place.”

She added: “I think what is right is that the Government looks at every possibility of an attack and looks at dealing with that, at protecting the country from potential attacks but obviously, crucially, the work that is done in day in day out, together with the police, to prevent attacks, they do a very good job.

“We can never be complacent, we must always look to see if there is something more that we can be doing.”

The Telegraphdisclosed on Saturday how William Shawcross, the chairman of the Charity Commission, had warned that money intended to ease the refugee crisis was “undoubtedly” going to extremist groups.

Mr Shawcross said: “A lot of money is raised that goes to Syria, some of it undoubtedly goes to extremist groups... It is very hard for all organisations to determine that.”

The Commission, which was unable to say how much money was going to extremists, is now so worried that it has issued guidance to charities on how to help charities deal wit the problem.

Peter Clarke, the former head of anti-terrorism at the Metropolitan Police who sits on the board of the Commission, said donors’ cash could fall into terrorists’ hands once it was moved from the UK to Syria, or the surrounding areas.

He told The Telegraph: “We know there is some abuse of charities by extremist mterrorist organisations but the likelihood is the full extent of this will never be known. What we have got to do is try to stop as much of it as we possibly can.”

The Disasters Emergency Committee, which represents the country’s 14 biggest charities, has raised £20million since the launch of its Syria Crisis Appeal in March.

However the Committee said that it was unable to guarantee that cash raised from fund-raising by charities was not falling into the hands of terrorists.

A spokesman said: “It is never possible to entirely eliminate such risks if you wish to help those in greatest need but DEC members take all reasonable steps to avoid, uncover and minimise such losses.”